Enable help generation for some git-cl subcommands

The subcommends "description" and "diff" in git-cl could not print the
help message even if --help was provided.

BUG= 473406

Review URL: https://codereview.chromium.org/1053103002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294663 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -1569,6 +1569,7 @@ def CMDcomments(parser, args):
 
 def CMDdescription(parser, args):
   """Brings up the editor for the current CL's description."""
+  parser.parse_args(args)
   cl = Changelist()
   if not cl.GetIssue():
     DieWithError('This branch has no associated changelist.')
@@ -2897,6 +2898,7 @@ def CMDset_close(parser, args):
 
 def CMDdiff(parser, args):
   """shows differences between local tree and last upload."""
+  parser.parse_args(args)
   cl = Changelist()
   issue = cl.GetIssue()
   branch = cl.GetBranch()